Unhextrium
Unhextrium, Uht, is the temporary name for element 163. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uht may form during neutron star mergers. NUCLEAR PROPERTIES At least one model exists predicting the half-lives and decay modes for nuclides up to Z = 175 and N = 333(1), which includes isotopes Uhb 495 and lighter. It is helpful to view p 18 of Ref. 1, which maps predicted half-lives of nuclides in this region, and p 15, which maps principal decay modes. These maps are the result of large extrapolations from what can be tested, though. Half-lives are accurate only to within three orders of magnitude and decay modes give no information about competing minor decay modes. Results given in this article should be regarded as tentative. Ref. 1 predicts a band of nuclides ranging from Uhb 435 to Uhb 471. As neutron count increases in this band, half-life increases, reaching a maximum exceeding a second. Principal decay mode shifts from fission, to alpha emission, then to beta emission. This pattern is expected, given the predicted neutron shell closure at N = 308. However, there is also a band of alpha-decaying isotopes with millisecond-scale half-lives predicted to lie between Uhb 472 and Uhb 481, a zone which should be strongly destabilized by the N = 308 closure. A neutron shell closure has also been predicted to occur at N = 318(2) and a proton shell closure has been predicted at Z = 154(3). The long lives reported for Uhb 472 to Uhb 481 may indicate effects of those two closures. Beyond Uhb 495, predicting nuclear properties is largely guesswork. At least two sets of predictions exist for location of the neutron dripline up to Z = 175(3),(4). These two indicate that the dripline occurs between Uhb 555 and Uhb 585. Neutron shell closures are also predicted to occur at N = 370(3) and 406(5). Ref. 5 also includes predicted structural correction energies provided by shell closures at N = 406 and Z = 164. Applying the Z = 164 correction at full strength predicts that Uhb 458 through Uhb 495 have fission half-lives exceeding 0.001 sec, which is inconsistent with the much higher-quality results of Ref. 1. Multiplying the Z = 164 corrective energy of Ref. 5 by 0.7 makes the two sets of data consistent with each other. With this adjustment, beta-decay can be expected to predominate in isotopes between Uhb 540 and Uhb 585, with a significant beta-decay branch occurring as low as Uhb 530. The band from Uhb 540 to Uhb 555 is particularly important, since it lies below the lower neutron dripline prediction, strongly implying that these isotopes will beta decay. Although it cannot be quantified, the N = 370 closure should produce a second band of beta-decaying isotopes which may extend as far as Uhb 516 to Uhb 536. FORMATION Polar jets emanating from young neutron stars and black holes function like giant, sloppy particle accelerators. It is possible for fusion or multinucleon transfer reactions to produce all isotopes of Uhb, but only in atoms-per star quantities. This section addresses possible isotopes which can form in quantity. Material originally found 800-1000 m beneath the surface is expected to be ejected from a neutron star when it disintegrates during a merger. This material will consist of nuclides at or near the neutron dripline and having a proton count which may go as high as Z = 170. A zone of beta-decaying nuclides extending from the dripline to Uhb extends as low as A = 547, implying that the isotopes between Uhb 540 and Uhb 555 are likely to form in quantity (taking b+x*n decay into account), and that isotopes between Uhb 556 and Uhb 585 are possible, depending on where the dripline actually occurs. These nuclei will have millisecond-scale half-lives. It is unlikely that neutron capture can produce more than an atom or two per star of nuclides heavier than A = 350. ATOMIC PROPERTIES Uhb is predicted to be a transition metal (d block), although it is never occurs in environments cool enough to have chemistry. Of more importance, its last (1s) ionization energy appears to be in the range 680 - 790 keV, meaning that bare nuclei are abundant only at temperatures above 5 gK. Electron configuration predicted for Uhb takes account of finite nuclear size, Nuclear shape is neither considered nor dismissed explicitly. Nuclear shape may have some effect on electron structure. REFERENCES 1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011. 2. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics.42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105. 3. “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013. 4. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061 5. "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v.68, no. 7, pp 1133-1137; 2005. (02-14-20) (01-02-20)